Filters used in communication devices such as mobile phones are required to have low loss in a passband, and to exhibit a high degree of suppression outside the passband. In recent years, the use of the direct conversion method removes an intermediate filter (IF filter), and further requires the high degree of suppression. Exemplary acoustic wave filters used as a filter are a SAW (Surface Acoustic Wave) filter and an FBAR (Film Bulk Acoustic Resonator) filter. However, the filter behaves as a capacitor at frequencies far away from the passband. Thus, the degree of suppression degrades at frequencies outside the passband, especially in a high frequency band.
A coupled multi-mode acoustic wave filter, which is formed by acoustically connecting SAW resonators or piezoelectric thin film resonators, is used to increase the degree of suppression in a wide frequency band. There is disclosed a technique that connects a resonator in series or in parallel to a DMS (Double Mode SAW) filter in Japanese Patent Application Publication Nos. 6-260876, 2001-230657 and 10-93381 (Patent Documents 1, 2 and 3). Moreover, Patent Document 3 discloses a technique that reduces an inductance of a bonding wire to improve the degree of suppression in a high frequency band.
However, conventional techniques are mainly directed to improve the degree of suppression around the passband, and have difficulty in improving the degree of suppression in a wide band, especially in a high frequency band. In addition, the reduction of the inductance disclosed in Patent Document 3 does not improve the degree of suppression sufficiently.